In the product of carbon-carbon composite articles, a preform is made with carbon fibers or carbon fiber precursors, in textile form or in the form of loose fibers, and the resulting fibrous matrix is densified, typically, by filling it with a resin which is then carbonized and/or by filling it with chemical vapor. The densification process is generally repeated until the pores in the preform are so narrow that further penetration by densifying agents is impractical.
Antioxidant treatments are required to protect non-friction surfaces of carbon-carbon composite brake friction materials, due to the high operating temperatures of braking systems utilizing these materials. Oxidation may be minimized by a technological process of applying anti-oxidation (“AO”) solution to the non-friction surfaces of the brake discs. Unfortunately, the most common AO solutions have a tendency to migrate through the porosity of the carbon-carbon friction material in the presence of atmospheric humidity. This migration leads to contamination by the AO solution of the friction surfaces of the carbon-carbon composite brake materials, thus decreasing the friction properties of the brake.
Combinations of phosphoric acid and various metal phosphates are commonly used for such antioxidant treatments. Unfortunately, these same materials have adverse effect on braking effectiveness. Specifically, they lower friction coefficients of the carbon-carbon composite materials to which they are applied. While this is not a problem on the non-friction surfaces of the brake discs, it is very much a problem when the antioxidant material contacts the friction surfaces thereof. Because conventional antioxidant treatments are virtually invisible on carbon-carbon composites in their cured state, accidental application thereof to the friction surface can go undetected, resulting in adverse performance of the brake friction material.
Currently employed measures to prevent AO migration include (1) limiting the amount of phosphoric acid in the AO solution and (s) using high AO char temperatures. However, both of these methods limit the anti-oxidation effectiveness of the AO solution, and they create technological problems during the manufacturing process.
U.S. Pat. No. 7,160,618 relates to an AO system which is resistant to humid migration. In the present invention, in contrast, migration is avoided no matter what AO system is used. U.S. Pat. No. 7,118,805 and US 2007/0218208 similarly relate to formulating the AO system in such a way that migration would not occur. The present invention allows the use of any AO system. US 2007/0199626 discusses, in paragraphs [0030] to [0032], the established art of creating a carbon-carbon composite material. The use of various resins to densify a precursor matrix is well known in industry, but is not widely practiced because the resultant compose density tends to be low. The low density occurs because carbonizing the resins closes the porosity in the composite body, thereby inhibiting subsequent further densification of the composite body.